There have been a variety of lighted signs which use a lamp in combination with a cover screen to provide a multi-color display such as the red and white exit sign used in buildings. Such displays present a visible image at all times even when the lamp is not energized.
The development of multi-color electroluminescent displays has a long history with much of its early development directed toward color television. The displays developed have been based on thin film technology which either provides multiple arrays of coplanar pixels or arrays of non-coplanar phosphor elements. In either case, the phosphor elements have characteristics which emit at different wavelengths in the visible spectrum.
Early multi-color display patents such as U.S. Pat. No. 2,925,532 teach employing a planar array of discrete phosphor regions which reside between two sets of spaced apart strip conductors. The strips in one set of conductors are normal to the strips in the other set of conductors. This crossed relationship allows individual phosphor regions to be selectively activated. U.S. Pat. No. 5,047,686 teaches creating coplanar regions of phosphor of different light emitting characteristics by selectively doping regions of a continuous layer of phosphor. U.S. Pat. No. 4,862,033 teaches another method for generating a coplanar array of discrete phosphor regions of distinct compositions so as to produce distinct frequencies of emitted light.
The '033 patent also discloses multi-color displays where the phosphor layers responsible for the emissions are not coplanar. There are a variety of patents which also teach multiple layers of phosphor; these include the following U.S. Patents:
U.S. Pat. No. 4,908,603; PA1 U.S. Pat. No. 5,043,715; PA1 U.S. Pat. No. 5,294,869; and PA1 U.S. Pat. No. 5,294,870.
The above described patents are limited in their teaching of multi-color thin film displays all of which require a large number of fine electrode leads to address the individual pixels which are responsible for the image. For thick films, the side-by-side phosphor regions cannot be individually addressed if the pixel size is small thereby limiting the resolution of the pattern which can be readily generated since thick film devices have course electrode leads. For the displays which employ non-coplanar phosphor elements, the intermediate layers required by the thick film technology will cause absorption of the light generated and thus a non-coplanar phosphor element will not be suitable for a thick film multi-color display.
While the limitations of printed multiple electrodes, particularly in the case of thick film displays place limits on the relative size of the distinct regions of the display, multiple electrodes in all cases would not be well suited to provide a marbled texture display.
Thus there is a need for a multi-color thick film display and method for making the same that will provide great flexibility in the colors displayed as well as to provide a uniform appearance in situations where the display is not energized.